The increasing emergence of new protein and peptide-based drugs makes necessary the development of rapid and sensitive methods to check consistency between and within batches of biotechnology pharmaceuticals to ensure product quality. Continuing work reported last year, we investigated the effects of insulin concentration on the hydrogen exchange of the two insulins, recombinant human and Lis/Pro in an attempt to rationalize the large difference in numbers of hydrogens exchanged .Additional work was performed on other mutant insulins to determine if small changes in the structure could give rise to large changes in the numbers of hydrogens exchanged.